Yaakov deeply loved Rachel, as is stated: "Yaakov worked seven years for Rachel. But he loved her so much, it seemed like no more than a few days" (Bereshit 29:20). It was not a carnal love but a holy and lofty one based on a profound harmony between their souls. The proof of this is that he was with Leah for an entire night, thinking that she was Rachel, and yet only in the morning did he discover the switch. His attraction to Rachel was clearly not bodily since he knew nothing about it and could thus be deceived for an entire night. Their bond was idealistic and spiritual. He loved her for what she was now.
Leah, however, was "disliked." This does not mean, G-d forbid, that Yaakov really hated her. Yaakov loved everyone. He even called the shepherds "my brothers" (ibid. 29:4). Leah was simply less loved than Rachel, as it says: "But he loved Rachel more than Leah" (ibid. 29:30). She feels "disliked" because she knew that as far as her husband was concerned, she was the subordinate wife. Yaakov, however, did slowly develop a deep love towards her, not as his wife, but as the mother of his children.
Yaakov's love for Rachel, despite all its sublimity, was not the ultimate in love. His love of Leah ended up deeper and more elevated. His love of Rachel, although it was a love in the depth of his soul, was a personal love. His love for Leah, however, centered on the creation of future generations and on the desire for continuance. By marrying and having children, one becomes "eternal" in this world since he is continued by his offspring. The heart of romantic love is, deep down, the love of and desire for continued existence. The Torah, by determining that the mitzvah is not marriage but procreation, makes abundantly clear that the purpose of marriage is not mutual pleasure and convenience but the bearing of children to ensure future generations. There is even an opinion that this command can be fulfilled with a concubine (The Rosh on Ketubot 1:12). G-d forbid that a person should act in such a shameful way, but the letter of the law indicates what is at the heart of the matter. A husband and wife must love and respect each other on a personal level, but all of this stems from a deeper purpose. Maran Ha-Rav Avraham Yitzchak Kook expresses this idea as: "Sexual impulses have been given to us in order to ensure the future of the world and of mankind."
Since Yaakov's love of Rachel was personal and unrelated to the propagation of future generations, their bond was infertile. Yaakov's spiritual romance must give way to a love which bears with it the responsibility of maintaining the continuity of mankind. Rachel demanded in painful exasperation: "Give me children or let me die!" (ibid. 30:1). She implored G-d to let her be fertile like her sister (See Rashi's second explanation and Onkelos on Bereshit 30:8), and Hashem finally opened her womb (ibid. 30:22). Her troubles, however, were not yet over. She died in childbirth when Binyamin was born (Ibid. 35:17-18). Rachel's temporary kingdom had to vacate its place for Leah's permanent kingdom to take over. The essential, romantic love was a preparation for the supreme state for them to "become one flesh" (Bereshit 2:24). This occurs when children are born, "for the child is created by both parents and in it their flesh becomes one and united" (Rashi ibd.). Both Rachel and Leah built the complete House of Israel.